Fuel cell assemblies convert a fuel and an oxidant to electricity. One type of fuel cell system employs a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as oxygen or air) to generate electricity. The PEM is a solid polymer electrolyte membrane that facilitates transfer of protons from an anode to a cathode in each individual fuel cell normally deployed in a fuel cell system.
In a typical fuel cell assembly (stack) within a fuel cell system, individual fuel cell plates include flow channels through which various reactants and cooling fluids flow. In subzero temperatures, water vapor in the fuel cell assembly may condense in the flow channels. Further, the condensate may form ice in the fuel cell assembly. The presence of condensate and ice may affect the performance of the fuel cell assembly and may also cause damage to the fuel cell assembly.
During typical operation of the fuel cell assembly in subzero temperatures, waste heat from the fuel cell reaction heats the fuel cell assembly and mitigates against vapor condensation and ice formation in the assembly. However, during a starting operation or low power operation of the fuel cell assembly in subzero temperatures, water vapor may condense and the condensate may form ice within the fuel cell assembly before the waste heat from the fuel cell reaction heats the fuel cell assembly.
Typical fuel cell assemblies are in cathode fluid communication with a compressor including surge control hardware and software. The compressor increases the pressure of a fluid flowing therethrough by reducing a volume of the fluid within the compressor. Increasing the pressure of the fluid increases the temperature of the fluid. The surge control hardware of the compressor mitigates against compressor surge, or the reverse flow of the fluid through the compressor, caused by a pressure drop or back pressure in the fuel cell assembly. Current fuel cell assemblies having compressor systems use a system bypass valve to reduce the amount of fluid caused to flow to the fuel cell assembly. The bypass valve facilitates obtaining a desired fluid pressure to mitigate against compressor surge. The fluid caused to flow through the bypass valve is purged to a vehicle exhaust system.
The fluid purged to the environment from the exhaust system of the fuel cell system is also a concern. Unconsumed hydrogen (H2) is the most important emission consideration from the fuel cell system of the vehicle. The hydrogen exiting the vehicle must be kept below a lower flammability limit (LFL) of approximately 4% molar concentration of hydrogen in air. If the exhaust from the fuel cell assembly is above 4% molar concentration of hydrogen, the fuel cell assembly can be operated at increased airflows to dilute the hydrogen in the exhaust stream. Operating the fuel cell assembly at an increased stack airflow may dry out the fuel cell assembly.
It would be desirable to produce a fuel cell system heated by a fluid in the fuel cell system during a starting operation to mitigate against vapor condensation and ice formation in a fuel cell assembly and to decrease a warm up time of the fuel cell system.